At present, concrete is the second-most used material by mankind behind water. The overwhelming success of concrete as a construction material is related to the widespread availability of its raw material components, to its relatively low cost and to its ease in application, being fluid and workable when freshly prepared and transforming into a hardened, competent rock-like material when cured.
Concrete is a composite mix of fine and coarse aggregates bound together by a cementitious binder. In the overwhelming majority of cases (>98%) the cementitious binder is based on Portland cement clinker interground with calcium sulfate. It is estimated that to date about 4 Gt/y of Portland clinker is being produced. Limestone and clay are the conventional clinker raw materials that are fired at 1450° C. to form the clinker. The firing process and the decomposition of limestone into CaO and CO2 amount to a typical combined CO2 emission of 0.8 t CO2/t clinker. Considering the scale of production it is estimated that 5-8% of man-made CO2 emissions are related to cement manufacture.
A common approach for lowering the environmental impact of cement production is the partial replacement of Portland clinker by supplementary cementitious materials (SCMs) in so-called blended cements. SCMs are usually low-cost by-products from other industries such as blast-furnace slags from steel production or fly ashes from coal combusted electricity production. Next to cutting industrial CO2 emissions and energy consumption by clinker production this approach enables valorisation of large volumes of by-products and avoids landfilling of wastes. Based on globally averaged clinker replacement levels it is estimated that currently about 520 Mt/y of materials are used as SCMs in cement and concrete products. Since most conventional, high-quality SCMs such as blast-furnace slags are practically entirely consumed, further incremental reductions of the environmental impact of cement production will need to come from new, alternative materials. The current object of invention is claimed to be one of these.
Since the supply of conventional high-quality SCMs is limited and fully utilized, one approach to further increase clinker replacement levels is to produce alternative SCMs such as thermally activated clays. A commonly shared property of SCM blended cements is their slower strength development compared to Portland cements. This and the related lower heat release is beneficial to some applications such as mass concrete. However for the cement producer who aims to meet pre-set strength requirements, slow strength development is a factor limiting the maximal clinker replacement level. In this respect, the key quality parameters for SCMs are reactivity and contribution to strength development. One way of enhancing the reactivity of potential SCMs of interest is thermal activation. Prior art learns that thermal treatments are mostly targeted at natural clays. Thermal activation of clays at temperatures between 550 and 800° C. can result in reactive SCMs that show acceptable early strength development at relatively high replacement levels of 20-40 wt. % of the binder (e.g. U.S. Pat. No. 5,788,762). However, the performance of the calcined clay SCMs was found to be uneven and to depend on (phase) composition. Calcination, and more specifically dehydroxylation, of clay minerals produces an amorphous material that can be very reactive as supplementary cementitious material. Metakaolin, produced by calcination of kaolins is known to be the preferred and most reactive among the activated clay minerals (He, C., Makovicky, E. and Osbaeck, B., ‘Thermal stability and pozzolanic activity of calcined kaolin’, Appl. Clay Sc. 9 (1994) 165-187.). Reactive SCMs can also be produced by calcining kaolinite containing residues from oil sand processing (at least 40 wt. % kaolinite) as described in GB2316333A. Other clay minerals may be variable in reactivity. Smectite clay minerals show considerable pozzolanic activity when properly calcined. In contrast, other common clay minerals such as illite and chlorite were shown to have little reactivity towards hydrated cement when calcined (Fernandez, R., Martirena, F. and Scrivener, K., ‘The origin of the pozzolanic activity of calcined clay minerals: a comparison between kaolinite, illite and montmorillonite’, Cement Concrete Res. 41 (2011) 113-122; Snellings, R., Mertens, G. and Elsen, J., ‘Supplementary cementitious materials’, Rev. Mineral. Geochem. 74 (2012) 211-278; Trumer, A., Ludwig, H.-M., ‘Investigations into the application of calcined clays as composite material in cement’ Zement-Kalk-Gips International 67 (2014) 52-57). Hence, the use of clays comprising mainly illite and chlorite was previously regarded as being unsuitable as SCM. A later invention describes cementitious binders existing of Portland clinker blended with a calcined clay and carbonate material (EP2253600A1). The beneficial “synergetic” effect on strength development was obtained for clays containing clay minerals belonging to the kaolin and smectite groups. In this case the thermal activation process should avoid a chemical reaction between the clay and carbonate materials.
In another patent (U.S. Pat. No. 4,737,191) a process is described to produce a hydraulic binder by the reaction of clay phases and calcium carbonate at temperatures of 700-900° C. An increased CO2 partial pressure is maintained to avoid the formation of free lime (CaO). Yet another patent application (WO98/28046) describes a thermo-chemical treatment of contaminated sediments consisting of a mixing with additives such as calcium oxides and heat treatment at elevated temperatures of 1150-1500° C. In this process an entirely molten slag is produced that can be used as supplementary cementitious material. Thus, the state-of-the-art describes the production of SCMs by thermal activation of clay materials. The presence of kaolinite and smectite group clay minerals in the original clay is desirable. Other clay minerals are perceived to be less suitable, even when calcined. Since kaolinite and smectite group clay minerals are less common in non-tropical regions, there is a need for alternatives in more temperate regions such as North and Western Europe.